The invention relates to an arrangement in a paper machine or similar, which includes a press section equipped with one or more press nips and a dryer section comprising a web-supporting closed web transfer, a vertical impingement dryer and one or more subsequent cylinder dryer groups. The invention relates particularly to impingement unit applications blowing directly to the web.
With increasing paper machine speeds the runnability of the machine becomes very critical unless measures are taken at the same time for improving runnability. Runnability can be improved up to a certain limit by maintaining a sufficient web tension by means of a speed difference between successive stages. Even this method will become exhausted at the stage when the paper quality starts to deteriorate.
Rising paper machine speeds have led to a tendency to preferably use a closed transfer from the press section to the dryer section, and, particularly in a multicylinder dryer, the single fabric run arrangement, as far as possible, even to the end of the cylinder dryer. These are used to get rid of fluttering and similar phenomena, which occur in the free web transfer. From the center roll of the press section the paper web can however be picked up to the dryer section using an open transfer.
A paper machine dryer section using merely a multicylinder dryer becomes fairly long at high, 30 m/s to 40 m/s, speeds. According to Finnish patent 102623 (WO 97/130131) and Finnish patent application 20002429, impingement dryers are used to replace dryer cylinders, particularly at the beginning of the dryer section, in which full steam pressure cannot be used in dryer cylinders or steam supply of the first cylinder is sometimes even completely closed. A wet paper web attaches to a hot cylinder surface due to which it is necessary to use a lower cylinder surface temperature, whereat drying capacity is lost.
In an impingement drying unit, in which impingement takes place directly against the paper web and not through the fabric, it is possible to use fairly high blowing temperatures, 250° C. to 700° C., and thus achieve a very efficient heating effect. The paper web is set to travel on top of a support fabric, which is supported in the blowing area by a set of rolls either in a straight run or with a large curvature radius. Suction/blow boxes are placed between the rolls for keeping the paper web against the support fabric.
According to patent application 20002429 (WO 02/36880), it is possible to spare the machine-directional length by using one or more vertical impingement units. The support fabric has in the vertical direction a notably long loop compared to its machine-directional dimension, at least in the dryer cylinder line. The support fabric remains under the paper web as regards blowing and consequently is not subjected to heat. On both sides of the loop generally there are impingement units, both of which thus have a drying length of even several meters. Keeping the paper web attached to the support fabric is ensured by using internal suction devices, which direct the suction effect to the paper web from inside via the support fabric. The side profile of the impingement surface is straight, slightly curved, possibly variably curved, in a shape of a broken line or a combination of these.
The impingement unit comprises a web arrangement that provides support for the paper web and a blowing chamber, which has a perforation on its web side flank for distributing air or other hot gas onto the blowing surface.
Space saving is realized also in such a case when the orientation of the unit deviates even remarkably from the vertical, as it will in any case be located in a space below or above the paper machine. On the other hand, a vertical construction has the advantage that the earth's gravity cannot disturb the attachment of the fabric to the support surface.
In a closed transfer, a great number of fabric loops composed of support fabrics are needed. As the number and total length of these increase, web break risks generally increase. Therefore, the optimization of their number and lengths is aimed at.
Although the above-mentioned known impingement solutions have provided improvements compared to the prior art technique related to runnability at high speeds and the machine size in the longitudinal direction, the situation has not been completely satisfactory. A simpler, yet a reliable concept is still required.
The bulk, in units of cm3/g, of paper is a significant quality factor for many paper grades. However, good bulk is in contradiction with the maximum press section dewatering, because achieving a high dry content after the press requires high nip pressures.
According to patent 102623, an impingement unit is located after the press section before the first dryer cylinder. Units blowing through a fabric according to the patent suffer from the blast air temperature limit, since the present drying fabrics cannot be stressed with blast air or steam hotter than 200° C. The construction becomes, however, relatively long, and the machine longitudinal saving is not notably achieved with simple solutions. With vertical impingement units according to patent application 20002429, remarkable savings are achieved much faster in the machine length. With the proposed solutions using vertical impingement units, the runnability is not better than today after the press section.